In the microscopic examination of specimens, microscopes of various types that are suitable for the particular intended application are used. For high magnifications and high-contrast depictions, electron-beam microscopes are particularly suitable, for example a transmission electron microscope (TEM), a scanning electronic microscope (SEM) or a confocal scanning electron microscope (CSEM), with which a sample is scanned in three dimensions with the focus of an electron beam.
When samples are examined by means of electron beams, whether with a TEM, SEM, or CSEM or by electron lithography, undesirable deposits of hydrocarbon compounds or water often occur on the surface of the specimen being examined. Three causes of this, and combinations thereof, can be identified:                the vacuum system;        device components, for example a movable sample holder, lubricants, O-rings, etc.; and        impurities already present on the specimen before the examination is carried out.        
The contamination products on the one hand interfere with the image and on the other hand can simulate specimen features, resulting in misinterpretation. In critical dimension (CD) measurements, which are performed at high magnification and therefore with a high beam intensity per unit area, contamination on the feature being measured means a change in the actual feature width. This therefore easily leads to incorrect measurements.
When a CD measurement is carried out on masks using a CSEM, destruction of the mask can occur as a result of light-absorbing contamination. In the case of so-called defect review CSEM, the decreased conductivity of materials in the contact holes after irradiation with electrons is also known to be a problem. The deposited hydrocarbons have an electrically insulating effect.
A number of possibilities for preventing or remedying the aforementioned disadvantages are already known. U.S. Pat. No. 6,077,417, for example, proposes the use of so-called microlens stacks to improve the electron illumination system. To remove contamination from the microlens stack, it is proposed to heat each microlens of the stack individually by delivering current. This is typically carried out at a temperature of approximately 200 degrees Celsius in order to prevent deposits. Impurities that have already become attached can be removed by occasionally heating the microlenses to 600 to 700 degrees Celsius.
U.S. Pat. No. 6,105,589 discloses a method for cleaning the specimen surface. In this, the specimen surface is cleaned by means of a plasma discharge. A plasma discharge device is provided for this purpose on the specimen chamber. It can contain air or a different oxygen/nitrogen mixture. Free oxygen radicals are generated by means of the plasma discharge. These free oxygen radicals disperse by convection inside the sample chamber, and thus also reach the surface of the specimen. There these free radicals are capable of removing hydrocarbons from the surface of the specimen, converting them into CO gas or water vapor.
U.S. Pat. No. 6,038,015 furthermore discloses a method for eliminating, from a mask, contamination that has a disruptive effect when irradiation with electron beams occurs. In this, the masking is scanned and a detector senses the electrons that are emitted at the location of a contamination product. Once the impurity has been detected in this fashion, it is then removed by a mask cleaning system. A laser beam or a locally delivered reactive gas is used to remove the contamination product. Once the contamination product has been removed in this fashion, the masking is placed onto the specimen that is to be examined.
To clean the surface of a semiconductor wafer, U.S. Pat. No. 4,752,668 discloses a method that operates with the aid of an excimer laser. For this, the wafer is positioned on a stage that is displaceable in a plane, and the excimer laser is focused onto those points at which material, in particular impurities, are to be removed. The laser beam is guided through a vacuum, and the impurity is carried off by way of the vacuum pumps.
A so-called cold trap is also often used in order to improve conditions during microscope operation, and thus to create a good capability for decreasing contamination even during microscope operation at high magnifications. It is known for this purpose, for example from U.S. Pat. No. 6,046,457, to place a metal ring, in particular a copper ring, in the vicinity of the specimen that is to be examined. This method promotes the condensation of contamination products on the cold trap. This method is not, however, widespread in microscope systems that are used in the semiconductor mass-production industry, since it involves a large maintenance outlay.